Method of heat treatment and apparatus therefor



RJM M T. G. M DQUGAL METHOD OF HEAT TREATMENT'AND APPARATUS THEREFOR 3 Sheets-Sheet I Filed Dec. 12, 1925 y my To 5. M DQUGAL 3333,35

v METHOD OF HEAT TREATMENT AND APPARATUS THEREFOR Filed Dec. 12, 1925 3 sheets-sheet 2 .5, w m K92] w g? w 5 gwuento'o TAINE snc DOUGAL 'r. e. M DOUGAL 1 851 EATMENT AND APPARATUS THEREFOR Filed Dec. 12, 1925 :s Sheets-Sheet 5 J :ML 7 4 v .Hnlu F I: a

6x E 5 2E nkwz w m ma D N a May 21, 1929.

Patented May 21-, 1929.

v UNITED STATES PATENT OFFICE.

'TAINE G. MCDOUGAL,OF FLINT, MICHIGAN, ASSIGNOR Tb A C SPARK PLUG COMPANY,

OIEFLINT, MICHIGAN, A COMPANY OF MICHIGAN.

METHOD OF HEAT TREATMENT AND APPARATUS THEREFOR.

Application filed December 12, 1925. Serial No. 75,046.

The method of heat treatment disclosed in this application may be used wherever it is desired to subject an article to relatively high temperatures in any of the various industrial processes as in the baking of enameled articles, heat-treating of metals, and the like. However, the invention has been developed in connection with the treatment of ceramic articles and in order that the scope of the invention maybe best understood a brief resume is given of prior processes employed in the burning of ceramic wares. It is to be understood that these processes are fairly typical of the processes employed in heattreating generally. I

According to the oldest of the methods employed in the firing of ceramic articles, the wares are placed in saggers or fire clay boxes that are stacked in the kiln. the charging entrance to which is then bricked up and the kiln fired. This process is necessarily slow in order to insure proper distribution of heat throughout the large mass of wares and saggcrs. Twenty four to one hundred forty four hours are required to complete a firing cycle. The procws is also subject to loss of wares from over-firing and under-firing because of lack of uniformity 1n heat distribution throughout the large refractory mass;

is expensive because of the necessity for maintaining a supply of saggers, these being subject to rapid deterioration in use; and is inetlicient because of the amount of labor involved in handling the ware and also. from the standpoint of heat, because of the large amount of heat absorbed by the saggers and kiln and lost upon their subsequent cooling in the intervals between the periodical firing of units. a

In an effort to overcome the deficiencies in the above process, a continuous process has been developed in which the ware is placed in fire clay saggers. piled high upon cars having refractory tops to protect the metallic running gear of the cars from the heat of the firing chamber. 'llhe cars thus loaded are passed through a long tunnel (from 220 to' 400 feet-in length) in which they are gradually heated to the required temperature followed by a gradual cooling. This method facilitates handling of the wares and rethis process there is also considerable loss from over-firing and under-firing resulting from uneven heat distribution throughout the mass of Wares and refractories,' and, While the kiln is continuously heated and consequently the heat loss in the kiln itself is much reduced as compared with the periodic type kiln, the saggers and the refractory car tops are subject to alternate heating and cooling and the heat lost through absorption by these large masses is very great.

as low as 6 hours, or even less, and a more uniform product is secured. When this arrangement of articles is used the inner; rows of wares, especially the lower portions farthest from the source of heat, are nevertheless partially shielded fronrdirect radiation of heat from the top and vertical walls of the kiln and, when the shapes treated are spark plug porcelains, suspension type insulators,-or the like, having portions intermediate between the source of heat and those other portions of the individual wares of greater width than the lower end portions, or are flat pieces such as tile or plates, the lower portions or undersldes of the wares are more or less elfectively shielded from direct radiation'from the kiln walls b the upper portions. The arrangement 0 articles is also such-as to interfere with the free circulation of hotgases about them. Consequentl that portion of an article of such shape shlelded from direct radiation and subject only to an impaired flow of hot gases must receive a large proportion of its=-heat by conduction from the hotter portions of the same article and by rera'diation from adjoining articles. But not only is the flowof heat into the waresthus obstructed but the large supporting mass of refractories which enters the kiln cold and on which the wares rest affords an excellent path for the conduction of heat away from the wares.

The Heat thus absorbed by the refractory car deck is lost upon its subsequent cooling.

cle alike, but with a lack of uniformity asbetween different parts of the same article, and where conduction of heat from the bottom portions of the wares is so great, considerable time is required for the passing of the wares through the kiln so that each part of each article may be brought up to the required temperature. At the same time the unequal heating of different portions of the same article sets up strains which may lead to fractures known to the-art as pre-burn cracks. The large quantity of heat lost by absorption by the car deck also results in a serious lower ing in efiiciency of the operation. Thus while the process of my prior patent constitutes a substantial improvement on previous practices in that variation in heat distribution be tween the individual articles is substantially eliminated, my present process constitutes a further improvement in that temperature varia'tions'betwe'en portions of the samearticle are minimized. I

In carrying out my improved process heat is directly and efiectively applied to the were on substantially all sides including top and bottom causing a practically uniform rise in temperature throughout the article and reducing the firing time to a minimum, this latter efiect permittin the thickness and consequentl amount 0 refractory material or kiln urniture used in supporting the wares in their travel through the kiln to be greatly reduced so that the amount of heat lost by ab sorption by the wares-supporting refractories becomes a minimum. 1

According to my improved process the wares are passed, preferably in single file,

through a tunnel of special formation, this tunnel havingtop, bottom, and side walls which may, if desired, be blended into each other according to any preferred configuration. A slot is provided in one of the walls of the/tunnel, preferably the bottom wall, the

slot being substantially filled by the refractories upon which the wares are supported during their passage through the tunnel. The width of the tops of the refractory supports should be less-than the greatest width of the wares so that portions of the wares overhang the bottom of the tunnel and. are subject to radiation therefrom as well as from the lower portions of the sides of'the tunnel. However, this last condition isnot absolutely essential, although it is essential that stationary heat reflecting surfaces be provided closely adjacent the lower portions of the articles so .as to effectively direct radiant heat upon the portions of the articles which, according to prior practices, were effectively shielded from direct radiation. In the embodiments of the invention herein disclosed these stationary heat reflecting surfaces are preferably constituted by portions of the bottom Wall of thekiln.

- the wares.

support is small owing to the fact that because of the small area of the upper portion of the support which contacts with the wares and which is exposed to the heat of the kiln, as well as the small mass of the support, this small portion of the carrying refractory soon attains about the same temperature as the wares and the heat exchange between wares and support is ver slight; a

When wares are passe through a kiln constructed as above outlined all parts of the articles are quickly brought up to the required temperature. about two hours is required to complete a ring cycle in comparison with the six hours or more required accordin to the process disclosed in my prior patent, the twenty four hours or more reuired by previous tunnel kiln practices, and \t 1e twenty four to one hundred and forty four hours required by the periodic process. In this relatively short time (2 hours or slightly more) heat can penetrate but a small dis tance down through the thickness of refractory material of the character composing the travelling wares-supports so that the thickness of refractory required for the protection of the metallic running gear is greatly reduced. Thus whereas 18 inches of refractory protection, and in mostcases more, was presequent cooling withouthaving accomplished any productive result.

T have also found that firing by this process results in a very even application of heat by vradiation and convection as distributed from top to bottom of the ceramic piece'as it is disposed on the carrying means and as it advances toward the maximum temperature of the hot zone. The even heating of the piece eliminates strains which would otherwise be set up, and reduces liability of fractures. At the same time a more uniform glaze is obtained.

It isobvious that my process and the apparatus used for carr mg it out may be variously modified. Tius the process is as applicable to mufile type kilns or electrically 'heated kilns as it is to those in which com.-

bustion takes place in the same chamber with It should also be noted that the process is not limited to cases in which but a single column or file of articles is advanced through the kiln, although this .is to be preferred, but a plurality of columns spaced across the width of the tunnel and preferably separated by stationar heat reflecting portions of the bottom of t e kiln may be passed through a single tunnel provided the condition is fulfilled that the individual rows of articles are subject to direct radiation and are thoroughly bathed in the hot gasesduring' their passage through the kiln. It is also possible to arrange the articles in tiers provided they are so spaced as to permit direct application of heat by radiation and convection as before. In such event, however, for greatest efliciency the condition should be observed that the width of the supporting bodies should be considerably less than the greatest width of the pieces to be fired.

Reference is now made to the drawings in which several embodiments of my invention are disclosed.

In the drawings: Figures 1 and 1 arecomplementary longitudinal views of one form of' my improved tunnel kiln, portions of the tunnel being shown in section. The figures show complementary halves of a complete kiln.

Figure 2 is a section on line 2-2 of Figurel.

Figure 3 is a section on line 33 of Figure 1. 7

Figures 4, 5, 6, and '7 are views taken respectively on-lines H, 57-5, 66, and 77 1 of Figure 1.

Figures 8 and 9 are detail views showing the conveying means with articles mounted thereon.

Figure 10 is a cross section through'the firing chamber of a kiln or slightly modified form.

Figure 11 is a cross section through a kiln whic' is constructed to treat a number of columns of Wares.

Figure 12 shows a typical cross section of the preheating zone of a kiln used in firing fiat pieces of ware, such as tile.

Figure 13 is a cross section of the preheating zone'of a kiln designed for the treatment of sus ension insulators or other similarly shape pieces of wars.

Figures 14 and 15 are respectively cross sections and longitudinal sections of portions of a kiln designed for the treatment of flat ceramic wares arranged for the glost burn.

I shall first describe the form of kiln illustrated in Figures 1 to 9. This kiln is of the sired. In a direct-fired kiln properly mixed gaseous or liquid fueland air enters the high temperature zone B through openings 20,

perature zone B, the Wares pass through and under a bafiiearch 22 into the cooling zone C where they are first subject to indirect air cooling in the portion 26 of the kiln followed by indirect water cooling in the portion 28 and finally, if desired, by direct air cooling in the portion 29. The details of the stages of heat treatment are set out more fully in mp/lprior patent above referred to.

y present invention is characterized by the construction of the tunnel, particularly in the heating zones, and the cooperating conveying means.

n igures 8 and 9 l have shown in detail the construction of the conveying means with the wares mounted thereon. eterring to these figures, 30 indicates a refractory mass for supporting the wares and corresponding to the car deck used in present practices. The

support 30 is provided with integral upwardly extending projections 32 carrying pins 33 upon which are mounted the wares to be treated. While I have chosen to show'the pre' erred to provlde them with, metallic shoes 36 to take the wear and pushing compression and to reduce friction. I have provided the shoes 36 with lugs 38 which are engaged by lugs or dogs on the chain 40 which is actuated by motor as through any suitable connections. If preferred, shoes 36 may be furnished with rollers or ma travel over stationary rollers to reduce rictlon; and the tunnel may be inclined so that gravity alone will efiect movement of the articles through the tunnel. The feature of gravity feed of wares throu h a tunnel kiln is described and claimed in my copendmg appli- Sand'seals may also be rovided between the conveying means and t e tunnel if desired.

However, it is to be emphasized that no particular form of conveying means is essential tomy invention.

The conveying devices with the wares mounted thereon travel through the tunnel from right to left as viewed in Figures 1 and 1'.

In Figure 2 I have shown a section through a portion of the preheating zone near t e cation, Serial No. 283,836, filed June 8, 1928.

' the insulator roject over the bottom 46 of the 7 tunnel entrance. In this portion of the tunnel the ware is given a preliminary heating by the gases passing outwardly from the 0Ombli13Stl0n chamber or high temperature zone .Figure 3 is a section through a hotter por-- tion of the preheating zone and it will be greatest width of the article, in this case the spark plug insulator 34, so that portionsof tunnel there subjecting the underside thereof to radiation from the bottom of the tunnel and the lower portions of the side walls. It will also be noted that the insulator" is supported in thetunnel so as to be freely-and o enly exposed on practically all sides to the ot ases travelling through the tunnel. At the junction of the side portions 44 and the bottom 46 troughs 47 are provided. These trou hs serve to intercept unburned particles 0 fuel and the like, preventing them from reaching the wares. Figure 4 is a section through a portion of the high temperature zone. As before stated pre-mixed fuel and air is. projected into the tunnel through passages 20 raising the walls of the tunnel to a high temperature and bathing the insulator 34 in hot gases. This portion of the tunnel is also provided with a top wall 43, side walls 44, and bottom wall 46. As in Figure 3 portions of the insulator 34 overhang portions of'the bottom,

and I have preferably'formed these portions 46 of the bottom of upwardly diverging configuration so as to more effectively subject the lower portions of the insulator to heat radiated therefrom. As before, the in-, sulator is freely exposed to contact with the hot gases in the tunnel. It will be noted that the tunnel has am enlarged cross section at this point, this being required for combustion purposes with such fuel as oil..

Figure 5 is a section through the baflie which separates the high temperature zone from the cooling zone. As shown in this figure the tunnel opening is designed to rather closely conform to the shape of the article treated so as to prevent the passage of'hot gases from the high temperature zone into the cooling zone. s

Figure 6 is a sectional view through the indirect air cooling zone, both the insulator and a portion of the support being freely exposed to the cooling actiop.

ciently heated b Figure 7 is a similar view through the indirect water-cooled zone.-

It will now be apparentthat I have provided in the princlpal heatin portions of the tunnel, means for heating t ie under porlZlOIlS of the treated articles which in prior constructions have been shielded by the upper or middle portions of the article and inefiiconduction through the upper portions 0 the wares and to some extent by convection and by reradiation from surrounding articles. This means consists, first, in the provision of radiating surfaces forming part of the walls of the tunnel so situated as to directly project heat up 'n the under portions of the wares. Specifitilly, I have so formed the support for the wares that the portions of the wares which are of reater diameter overhang portions ofthe ottom wall of the tunnel.

dividual insulators in an exposed position in the tunnel they are efl'ectively bathed in the hot gases which impart to them a portion of their heat.

It is apparent that this heating means would be equally effective if the tunnel were of the mufller type. The rays of heat would be projected in the same manner from the heated walls of the tunnel and the articles would be subjected to the convection cur rents characteristic of this type of furnace.

While I have restricted the special configuration of kiln to the high,temperature zone and a portion only of the preheating zone it is apparent that if desired it maybe applied throughout the length of the latter zone. Also, if desired, the same idea may be made use of in any or all of the cooling zones to effect such variation in the rate of cooling of the different portions of the articles and support as may seem desirable.

In Figure 10 I- have shown a section through the hi h temperature zone of a modified form of (iln. This differs from preceding forms in that the firing chamber conforms more closely in cross section to the shape of the article treated so that the heat radlated from the walls has but a short distance to travel to strike the article. A chamber of this size is permissible where the character of fuel is such that the volumeof combustion products is small as in the case of oxy-acetylene fuel,- or where an electrical source of heat is used.

It is well known that the high tem eratures prevailing in the kiln have a ten ency to form a melted glassy surface on the refractor forming the tunnelwalls. This is especially so when glazed articles are fired because the glaze materials partially volatilize from the ware and are deposited on the said walls. With the walls thus provided with a smooth unbroken heat reflecting surface it is possible to direct the rays of heat where desired by giving the walls of the tunnel the proper contour. Thus the lower portion of the walls of the firing chamber may be made of parabolic shape as shown at 51 111 Figure 10 to focus the rays of heat upon the object.

The refractory block 52, constituting the top of the firing chamber is also shown offset as at 54 to cooperate with thecorrespondingly shaped block 56 forming the lower portion of the firing chamber so as to prevent shlfting of the position of the blocks.

Fi ure 11 shows a further modification run several lines of ware through a single tunnel. In this form plates 36 carry supports upon which the ware 34-. is mounted as before. Partitions 60, of refractory material, se arate the lines of supports 30 and while these partitions may be fixed to the base of the kiln and theseparate lines of ware driven from a common propellin means located at oneend orthe other of the iln and operating upon chains or cams or the like having driving connection with the supports, I have preferred to illustrate in this figure a construction in which a transverse shaft 62 is journailed in arms 63 projecting downwardly hot n shghtly less degree, to direct radiation,

from the lates 36, the shaft having rollers 64; thereon neath the I respective shoes and running upon tracks 66. I have shown the central track provided with flanges 68 to hold theparts in alinement. The shaft 62 also bears rollers 70 located beneath the partitions 60 and serving as a movable support for thesepartitions, an E-beam 72 preferably being interposed between the rollers and the parp I v I Where I have used the term an in the titions to take the bending strains. i With this construction it will of course be necessary to provide means at the ends of the kiln for preventing endwise movement of the partitions 60 so that the upper surfaces of these partitions may serve as stationary heat re-' fleeting surfaces to assist in directing radiant heat upon the lowerportions of the wares. It will also probably be'found to be desirable toprovide sand seals at the ends of the plates 36 to furtherprotect the moving parts from the heat of the furnace. This construction may be regarded as merely an amplification of the slngle file or line method of handling the wares shown in the preceding views, for each of the files or lines of wares is freely and 0 only exposed on all sides to the gases o the kiln and also, although to from the kiln walls.

Figure 12 shows a typical cross section in a preheating zone such as would be used in woes of ware, such as tile, a piece of 'w ch is indicated at 80. The width of the top pend upon ieces .of ware orees arising from the over-hanging the strength of the individual 'in resisting, the cantilever parts of the travelling conveyor 82 will de of the piece. Flat pieces of ware may be burned in this way either for bisque or glost firing. The carrying means or conveyor 82 is shown as merely sliding through on the more or less refractory bottom of the kiln. The refractory carrying device or car introduced at the charging end of the kiln may be propelled by some suitable means andin turn ,force the line or train of cars ahead of it through the kiln. Figure 13 is a cross sectionthrough the preheating zone of a kiln designed for the treatment of suspension type insulators, or the like, one of which is shown at 90. The shoe 36 in this case carries a steel block or member 92 provided with wheels 94 which run upon a track 96.

The construction shown in Figure 13 may also be usedas an enamelling oven, articles, such as kitchen-ware, taking the place of the insulator shown at 90. v 1

Figures 14 and 15 are fragmentary views I showing the arrangement of flat ceramic wares which may be resorted to for the glost specification and claims I have used it in a general sense as a supporter of-combustion and it is to be understood that the phrase pre-mixed fuel and air should be construed to cover such combustible mixtures as oxyacetylene or the like, wherein the chief active ingredient of air is used for supporting combustion.

I claim:

l. The method of burning ceramic wares which consists in mounting the article to be treated in a kiln in ositlon to be exposed on substantially all sides to direct radiation from the walls of the kiln and applying heat to the said walls.

2. The method of burning ceramic wares -which consists in mounting the article to be treated in a kiln in osition to be freel exposed on substantial y all sides to the irect action of hot gases as well as to direct radiation from the walls of the kiln and applying heat to the kiln by burning prermxe air and fuel in said kiln in direct contact with said article.

.3. The method of burning ceramic wares which consists in applying eat to a tunnel kiln by burning pro-mixed fuel and air thereconstructed with top', bottom,

" projecting over the said slotted .wafi.

6. A tunnel kiln, the heating zone of which.

in, thus effectively heating the walls thereof and causing a circulation of hot gases therein, and passing wares in single file therethrough while openly exposed on substantially all. sides to direct radiationfrom the said walls as well as to the action of the hot gases. i

' 4. That improvement in the art of burn-' ing ceramic wares which consists in passing .the wares. 1n slngle file through a tunnel n the course of which the wares are subjected is in the form of asubstantially closed tube,

' said tube being provided with a narrow lonspective ieces which they su' port. 8, Int e combination as de ned barclaim v 7 said tunnel igg ing gitudinal slot, and a refractory support for v ares substantially closing said slot and varranged to travel through said tunnel, the mass of said support being small compared with the mass of the walls of said tunnel so as to reduce the amount of heat absorbed by said travelling support.

7. The combination of a tunnel kiln, means for heating thekiln, supporting devices upon which the wares are mounted,.said supportmg devices being adapted to move through the kiln and including refractory portions exposed to the heat of the kiln and metallic support ng parts remote therefrom, the width of the tops .of said supporting devices which are directly exposed to the heat of the kiln being considerably less than the width of the'largest horizontal cross section of V N ovided with ,Qadiatmg surfaces clo'sel flanking the said 'wares. 9. The combination of a tunnel kiln, means for heating the kiln, supporting devices upon which the ,wares are mounted, said sup orting dev'wes being adapted to be moved tlirough the kiln, and including metallic bottom pore tions remote from the combustion chamber and togi'j iortipns directl exposed to the heat.

of the 0 reduced imensions com ared with the dimensions of the wares-to e sect a reduction in the amount of heat passingfrom' the wares to the said supporting devices.

10.' In combination, a tunnel kiln, a refractory wares carrying means, said kiln being provided with a slot through which the wares carryin means is adapted to travel, wares mounte upon said carrymg means and openly exposed imthe tunnel, portions of the I walls-of the "tuna ta adjacent the said slot being 'shaped to roughly conform 1n contour tothe exposed surfaces of the wares to effectively heat the latter. 1 11. A tunnel kiln having the firing zone thereof formed with top wall, bottom wall,

and side walls, the bottom wall being provided with a slot for receiving the travelling.

supports for the wares to be treated, the portions of the bottom wall adjacent the slot diverging upwardly. I

12. In the combination as defined in claim 11, longitudinally extending troughs being provided-at the junction of the side walls an the bottom wall.

'13. A travelling sup ort for ceramic wares during their passage t rough a tunnel kiln,

comprising metallic supporting members, a refractory body portion supported on said members, reduced 7 portions extending upwardly from the body portion and adapted for exposure tothe heat of the furnace, and means for supporting the were upon said projecting portions. I y

14; In the structure as defined by claim 13, said reduced portions being provided with pins of refractory material for entering sockets in the ware.

15'. Asupport of refractory material upon upper portion thereof which is directly exposed to the heat of the kiln of reduced width, the lower portions of the support serving to diffuse the heat.

16. A'tunnel kiln having, a heating zone comprisingtop, bottom and side walls, said bottom wall being provided with a slot therein, conveying means for transporting wares through-said tunnel, said conveying means including refractory portions extending into said slot and adapted to support wares within which ceramic wares are mounted during theirpassage through a tunnel kiln having the said tunnel so that the wares are exposed to direct radiation from all of the walls of said tunnel facilitatin 'rapid and uniform heat- 1n the said re actor portions being of su stantially less widtli than the greatest width, of the wares carried thereb I 17. the combination as define in claim 16, said refractory, portions and the walls of said'slot co-operating to substantially seal said slot to prevent the escape of heat.

18.A tunnel kilnhaving a heating zone and a cooling zone, a bafile between said zones, a carrier for transporting wares through said kiln in single file, said baflle being provided with a passage therethrough conformin closelyin shape to the shape of the wares an carrier in cross-section to prevent the escape of heat from the heating zone to the cooling zone.

19. The combination of a tunnel kiln pro- I. vided with a longitudinal slot, a carrier for wares formed of refractory material and 0ccupying said slot and having reduced portions exposed ithin said tunnel for supporting Wares therein.-

20. In the combination as defined in claim" 19, the Walls of said slot and the corresponding snrfacesof the carrier interfitting to prevent the projection of radiant heat out- 10 Wardly of the'tunnel.

21. The combination of a, tunnel kiln pro- Vided with a longitudinal slot, a carrier for Wares including metallic running gear remote from the interior qf the tunnel and refractory portions supported on said gear and exposed Withinsaid tunnelythe Walls of said slot and the corresponding surfaces of the carrier interfitting to prevent the projection f of radiant heat upon said running gear.

In testimony whereof I aflix my signature.

TALINE e; MQDOUGAL. 

